阅读说明：本技术 基于dsc分析的xlpe电缆绝缘老化程度判定方法 (XLPE cable insulation aging degree determination method based on DSC analysis ) 是由 刘飞 江平开 于 2021-01-12 设计创作，主要内容包括：本发明公开了一种基于DSC分析的XLPE电缆绝缘老化程度判定方法,该判定方法首先在空气气氛下,采用DSC,测试不同温度下新电缆绝缘的氧化诱导时间,并计算电缆绝缘的活化能与外推工作温度下的寿命。其次,在氮气气氛下,分别测试新电缆和现场老化电缆绝缘的DSC升温曲线,应用Kobayashi模型,计算电缆绝缘的热历史温度和时间。然后根据时温等效原理,将老化电缆热历史时间折算为新电缆热历史温度值下的时间,并减去新电缆的热历史时间,以消除电缆制造时所产生的热历史；将修正后的老化电缆热历史时间进一步折算为工作温度值下的老化时间,把工作温度下的老化时间与寿命的比值作为老化状态表征参数。(The invention discloses a method for judging the insulation aging degree of an XLPE cable based on DSC analysis. And secondly, respectively testing DSC temperature rise curves of the new cable insulation and the field-aged cable insulation under the nitrogen atmosphere, and calculating the thermal history temperature and time of the cable insulation by applying a Kobayashi model. Then according to a time-temperature equivalent principle, converting the thermal history time of the aged cable into the time of a new cable thermal history temperature value, and subtracting the thermal history time of the new cable to eliminate the thermal history generated during cable manufacturing; and further converting the corrected thermal history time of the aged cable into the aging time at the working temperature value, and taking the ratio of the aging time at the working temperature to the service life as an aging state characterization parameter.)

1. A method for judging the insulation aging degree of an XLPE cable based on DSC analysis is characterized by comprising the following steps:

s1: testing the oxidation induction time of the new XLPE cable insulation at different experimental temperatures by adopting DSC, calculating the activation energy of the new XLPE cable insulation according to an Allen-nius equation, and extrapolating the service life of the new XLPE cable insulation at the working temperature;

s2: respectively testing DSC temperature rise curves of new XLPE cable insulation and field XLPE cable insulation, and respectively calculating equivalent thermal history parameters of the new XLPE cable insulation and the field XLPE cable insulation, including thermal history temperature and corresponding thermal history time, by applying a Kobayashi model;

s3: according to a time-temperature equivalent principle, converting the thermal history time of the insulation of the on-site XLPE cable into the time of the thermal history temperature value of the insulation of the new XLPE cable, and subtracting the thermal history time of the insulation of the new XLPE cable to obtain the corrected thermal history time of the insulation of the on-site XLPE cable;

s4: further converting the corrected thermal history time of the on-site XLPE cable insulation into aging time under a working temperature value; and taking the ratio of the aging time at the working temperature to the service life obtained in the step S1 as an aging state characterization parameter for judging the aging degree of the insulation of the XLPE cable on site.

2. The method for determining the insulation aging degree of an XLPE cable based on DSC analysis as claimed in claim 1, wherein step S1 further comprises:

taking n equal mass samples from the insulation of a new XLPE cable, and respectively carrying out isothermal OIT tests at n experimental temperatures, wherein n is a positive integer not less than 4;

introducing nitrogen to the sample at each experiment temperature, raising the temperature to the corresponding experiment temperature, switching to the air after keeping the temperature constant, and then carrying out isothermal OIT test;

the activation energy and lifetime calculations are based on the following arrhenius equation:

wherein T is the oxidation induction time, T is the experimental temperature, a is a constant, b ═ E/R is a constant, E is the activation energy, and R is the gas constant;

according to the oxidation induction time at n different experimental temperatures, a straight line corresponding to lnt-1/T relation is drawn, the activation energy E is obtained by linearly fitting the slope of the straight line, and the oxidation induction time at the working temperature, namely the service life, is extrapolated.

3. The method for determining the insulation aging degree of an XLPE cable based on DSC analysis as claimed in claim 1, wherein the shielding gas used in the test in step S2 is nitrogen, and the temperature rise range should cover the main melting zone of the cable insulation material.

4. The method for determining the degree of insulation aging of XLPE cable according to claim 1, wherein,

in step S2, the equivalent thermal history parameter is calculated according to the following equation:

T_P1(T，t)＝A₁(T)log(t)+B₁(T)

T_P2(T，t)＝A₂(T)log(t)+B₂(T)

A₁(T)＝a₁T+b₁

A₂(T)＝a₂T+b₂

B₁(T)＝c₁T+d₁

B₂(T)＝c₂T+d₂

wherein T is the thermal history time, and T is the thermal history temperature; t is_P1And T_P2Respectively the temperatures corresponding to melting peaks P1 and P2 on DSC heating curve, and T_P1＜T_P2＜T_mWherein, T_mIs the melting point; a is₁，a₂，b₁，b₂，c₁，c₂，d₁，d₂Are constants for the Kobayashi model.

5. The method for determining the degree of insulation aging of XLPE cable according to claim 1, wherein,

in step S3, the thermal history time of the field XLPE cable insulation is converted according to the following formula:

wherein T is the thermal history time before the insulation of the on-site XLPE cable is converted, T 'is the thermal history time after the conversion, T is the thermal history temperature of the insulation of the on-site XLPE cable, T' is the thermal history temperature value of the insulation of a new XLPE cable, E is activation energy, and R is a gas constant;

the thermal history time of the field XLPE cable insulation is further modified to:

t″＝t′-t₀

wherein t is₀Is the thermal history time of the new XLPE cable insulation.

6. The method for determining the degree of insulation aging of XLPE cable according to claim 1, wherein,

in step S4, the aging time of the field XLPE cable insulation is calculated according to the following formula:

wherein, t_sTo the operating temperature T_sAging time; e is activation energy, and R is a gas constant;

the aging state characterization parameters of field XLPE cable insulation are defined as follows:

A＝t_s/τ，A∈[0，1]

wherein tau is the service life of the new XLPE cable insulation at the working temperature.

7. An XLPE cable insulation degradation degree determination method based on DSC analysis as claimed in claim 6, wherein if a ═ 0 indicates that the field cable is not degraded; a ═ 1 represents the field cable end of life; if A is more than 0 and less than 1, the cable is in an aging state on site.

Technical Field

The invention provides a method for judging the insulation aging degree of an XLPE cable based on DSC analysis, belonging to the technical field of insulation aging evaluation of power equipment.

Background

Crosslinked polyethylene (XLPE) is currently the most widely used power cable insulation in power transmission and distribution networks at home and abroad. The XLPE cable inevitably undergoes insulation aging in the long-term operation process, is a main cause of cable line faults, even causes sudden large-area power failure, and brings immeasurable loss. Therefore, the on-site aging degree of the cable in operation can be mastered in time, and the method has important significance for safe operation of the power grid.

XLPE power cables typically have a design life of 30 years, with many cables laid early approaching retirement. Because the actual working conditions such as laying environment, operation load and the like of cables of different lines are different, the degree of insulation aging of the cables is unreasonable by using the operation years. Therefore, as early as eighties, developed countries have sampled faulty cables for analysis and detection, so as to determine the degree of insulation aging of other cables on the cable transportation line.

At present, laboratory diagnosis parameters of cable insulation are many and mainly divided into electric quantity parameters and non-electric quantity parameters, wherein the non-electric quantity parameters such as Oxidation Induction Time (OIT) are generally measured in an oxygen atmosphere, and the thermal-oxidation aging mechanism of the cable insulation is not consistent with the actual situation. In addition, a reliable insulation aging criterion is not established for various diagnostic parameters; before the cable insulation aging state is diagnosed according to the real-time data of the characteristic parameters, a cable insulation field aging rule based on the characteristic parameters needs to be established, so that long-term tracking, sampling and detection of cables in transit are needed, and the method is time-consuming. Therefore, it is necessary to provide a reliable and simple method for determining the insulation aging degree of an in-situ aged XLPE cable.

Disclosure of Invention

In order to overcome the defects of the prior art, the invention provides the XLPE cable insulation aging degree determination method based on DSC analysis, the determination method is simpler and more convenient, and a timely decision basis can be provided for the safe operation and maintenance of a cable line.

In order to achieve the purpose, the invention adopts the technical scheme that:

a method for judging the insulation aging degree of an XLPE cable based on DSC analysis comprises the following steps:

s1: testing the oxidation induction time of the new XLPE cable insulation at different experimental temperatures by adopting DSC, calculating the activation energy of the new XLPE cable insulation according to an Allen-nius equation, and extrapolating the service life of the new XLPE cable insulation at the working temperature;

s2: respectively testing DSC temperature rise curves of new XLPE cable insulation and field XLPE cable insulation, and respectively calculating equivalent thermal history parameters of the new XLPE cable insulation and the field XLPE cable insulation, including thermal history temperature and corresponding thermal history time, by applying a Kobayashi model;

s3: according to a time-temperature equivalent principle, converting the thermal history time of the insulation of the on-site XLPE cable into the time of the thermal history temperature value of the insulation of the new XLPE cable, and subtracting the thermal history time of the insulation of the new XLPE cable to obtain the corrected thermal history time of the insulation of the on-site XLPE cable;

s4: further converting the corrected thermal history time of the on-site XLPE cable insulation into aging time under a working temperature value; and taking the ratio of the aging time at the working temperature to the service life obtained in the step S1 as an aging state characterization parameter for judging the aging degree of the insulation of the XLPE cable on site.

Optionally, step S1 further includes:

taking n equal mass samples from the insulation of a new XLPE cable, and respectively carrying out isothermal OIT tests at n experimental temperatures, wherein n is a positive integer not less than 4;

introducing nitrogen to the sample at each experiment temperature, raising the temperature to the corresponding experiment temperature, switching to the air after keeping the temperature constant, and then carrying out isothermal OIT test;

the activation energy and lifetime calculations are based on the following arrhenius equation:

wherein T is the oxidation induction time, T is the experimental temperature, a is a constant, b ═ E/R is a constant, E is the activation energy, and R is the gas constant;

according to the oxidation induction time at n different experimental temperatures, a straight line corresponding to lnt-1/T relation is drawn, the activation energy E is obtained by linearly fitting the slope of the straight line, and the oxidation induction time at the working temperature, namely the service life, is extrapolated.

Optionally, the shielding gas used in the test in step S2 is nitrogen, and the temperature rise range should cover the main melting zone of the cable insulation material.

Alternatively, in step S2, the equivalent thermal history parameter is calculated according to the following equation:

T_P1(T，t)＝A₁(T)log(t)+B₁(T)

T_P2(T，t)＝A₂(T)log(t)+B₂(T)

A₁(T)＝a₁T+b₁

A₂(T)＝a₂T+b₂

B₁(T)＝c₁T+d₁

B₂(T)＝c₂T+d₂

wherein T is the thermal history time, and T is the thermal history temperature; t is_P1And T_P2Respectively the temperatures corresponding to melting peaks P1 and P2 on DSC heating curve, and T_P1＜T_P2＜T_mWherein, T_mIs the melting point; a is₁，a₂，b₁，b₂，c₁，c₂，d₁，d₂Are constants for the Kobayashi model.

Optionally, in step S3, the thermal history time of the field XLPE cable insulation is converted according to the following formula:

wherein T is the thermal history time before the insulation of the on-site XLPE cable is converted, T 'is the thermal history time after the conversion, T is the thermal history temperature of the insulation of the on-site XLPE cable, T' is the thermal history temperature value of the insulation of a new XLPE cable, E is activation energy, and R is a gas constant;

the thermal history time of the field XLPE cable insulation is further modified to:

t″＝t′-t₀

wherein t is₀Is the thermal history time of the new XLPE cable insulation.

Optionally, in step S4, the aging time of the insulation of the in-situ XLPE cable is calculated according to the following formula:

wherein, t_sTo the operating temperature T_sAging time; e is activation energy, and R is a gas constant;

the aging state characterization parameters of field XLPE cable insulation are defined as follows:

A＝t_s/τ，A∈[0，1]

wherein tau is the service life of the new XLPE cable insulation at the working temperature.

Optionally, if a ═ 0 indicates that the field cable is not aged; a ═ 1 represents the field cable end of life; if A is more than 0 and less than 1, the cable is in an aging state on site.

Compared with the prior art, the invention has the following beneficial effects:

by performing the OIT test in an air atmosphere, the actual thermo-oxidative aging process of the cable insulation is better simulated. In addition, the equivalent thermal history of the on-site aged cable is conjectured through DSC analysis, so that the unreasonableness of intuitively judging the aging state of the cable due to the difference of actual working conditions according to the aging age is avoided; and overcomes the defects of the traditional diagnosis method: before the cable insulation aging state is judged according to the current value of the characteristic parameter, a great deal of time is consumed for tracking, sampling and detecting the cable in transit for a long time so as to establish the cable insulation field aging trend based on the characteristic parameter. The invention provides a simple and rapid method for judging the insulation aging degree of the field aging XLPE cable.

Drawings

Fig. 1 is a flowchart illustrating steps of a method for determining an insulation aging degree of an XLPE cable based on DSC according to an embodiment of the present invention;

FIG. 2 is a lnt-1/T relationship diagram according to an embodiment of the present invention;

fig. 3 is a DSC temperature rise curve for cable insulation according to an embodiment of the present invention.

Detailed Description

The present invention will be described in detail with reference to specific examples. The following examples will assist those skilled in the art in further understanding the invention, but are not intended to limit the invention in any way. It should be noted that it would be obvious to those skilled in the art that various changes and modifications can be made without departing from the spirit of the invention. All falling within the scope of the present invention.

Fig. 1 to fig. 3 show a first embodiment of the method for determining the insulation aging degree of XLPE cable based on DSC analysis according to the present invention.

The embodiment of the invention provides an XLPE cable insulation aging degree determination method based on DSC analysis, which is used for determining the aging degree of on-site XLPE cable insulation and comprises the following steps:

s1: testing the Oxidation Induction Time (OIT) of the new XLPE cable insulation at different experimental temperatures by adopting DSC, calculating the activation energy of the new XLPE cable insulation according to an Arrhenius equation, and extrapolating the service life of the new XLPE cable insulation at the working temperature;

in the embodiment, the temperature range of the test is recommended to be 180-220 ℃, and the number of different temperatures is not less than 4.

Among them, dsc (differential Scanning calorimeter) is a differential Scanning calorimeter, which measures the relationship between temperature and heat flow related to the internal thermal transition of a material, and has a very wide application range, especially the development, performance detection, and quality control of the material. The properties of materials, such as glass transition temperature, cold crystallization, phase transition, melting, crystallization, product stability, solidification/crosslinking, oxidation induction period, etc., are all areas of differential scanning calorimetry research.

Activation energy refers to the energy required for a molecule to transition from a normal state to an active state in which chemical reactions readily occur. The difference between the average energy of the activated molecules and the average energy of the reactant molecules is the activation energy.

The Oxidation Induction Time (OIT) is a measure of the time during which the sample begins to undergo autocatalytic oxidation under high temperature (200 ℃) oxygen conditions, and is an indicator for evaluating the ability of the material to withstand thermal degradation during molding, storage, welding and use.

The new XLPE cable insulation can be considered as the as-shipped unused XLPE cable insulation, which is used to assist in determining the degree of aging of the XLPE cable insulation in the field.

S2: and respectively testing DSC temperature rise curves of the new XLPE cable and the field XLPE cable insulation, and respectively calculating equivalent thermal history parameters of the new XLPE cable insulation and the field XLPE cable insulation, including thermal history temperature and corresponding thermal history time, by applying a Kobayashi model.

The protective gas used in the test in step S2 is nitrogen, and the temperature rise range should cover the main melting zone of the cable insulation material.

In step S2, the equivalent thermal history parameters are calculated according to the following equation (i.e., Kobayashi model):

T_P1(T，t)＝A₁(T)log(t)+B₁(T)

T_P2(T，t)＝A₂(T)log(t)+B₂(T)

A₁(T)＝a₁T+b₁

A₂(T)＝a₂T+b₂

B₁(T)＝c₁T+d₁

B₂(T)＝c₂T+d₂

wherein T is the thermal history time, and T is the thermal history temperature; t is_P1And T_P2Respectively the temperatures corresponding to melting peaks P1 and P2 on DSC heating curve, and T_P1＜T_P2＜T_mWherein, T_mIs the melting point; a is₁，a₂，b₁，b₂，c₁，c₂，d₁，d₂Are constants for the Kobayashi model.

Among these, the availability of a1, a2, b1, b2, c1, c2, d1, d2 can be determined experimentally (DSC testing of samples treated at different temperatures for different times to obtain a series of T' s_P1And T_P2Values, each constant value calculated by linear fitting according to the model); or the values of a1, a2, b1, b2, c1, c2, d1 and d2 are directly adopted from the previous research data.

S3: in order to eliminate the thermal history generated during cable manufacturing, according to the time-temperature equivalent principle, the thermal history time of field XLPE cable insulation is converted into the time of a new XLPE cable insulation at a thermal history temperature value, and then the thermal history time of the new XLPE cable insulation is subtracted.

S4: and further converting the corrected thermal history time of the on-site XLPE cable into aging time at a working temperature value, and taking the ratio of the aging time at the working temperature to the service life obtained in the step S1 as an aging state characterization parameter for judging the aging degree of the insulation of the on-site XLPE cable.

Wherein, the step S1 further includes:

firstly, taking n equal mass samples from the insulation of a new XLPE cable, and respectively carrying out isothermal OIT tests at n experimental temperatures, wherein n is a positive integer not less than 4.

Introducing nitrogen to the sample at each experiment temperature, raising the temperature to the corresponding experiment temperature, switching to the air after keeping the temperature constant, and then carrying out isothermal OIT test;

the activation energy and lifetime calculations are based on the following arrhenius equation:

wherein T is the oxidation induction time, T is the experimental temperature, a is a constant, b ═ E/R is a constant, E is the activation energy, and R is the gas constant; the value of a is determined by linearly fitting the intercept of the line lnt-1/T. R is the gas constant of air, equal to 8.314J/(mol. K)

According to the oxidation induction time at n different experimental temperatures, a straight line corresponding to lnt-1/T relation is drawn, the activation energy E is obtained by linearly fitting the slope of the straight line, and the oxidation induction time at the working temperature, namely the service life, is extrapolated.

Wherein, the oxidation induction time of the lnt-1/T linear fitting straight line under the working temperature is externally pushed out, which is a conventional method and is not described again.

In this example, four equal mass samples were taken to perform isothermal OIT tests at four experimental temperatures, 180 ℃, 190 ℃, 200 ℃, and 210 ℃. The value range of the experimental temperature is 180-220 ℃.

The test is carried out according to GB/T19466.6-2009, nitrogen is introduced into a sample at each experimental temperature to raise the temperature of the sample to the corresponding experimental temperature, the sample is switched to air after being kept at a constant temperature, and then the isothermal OIT test is carried out. The isothermal OIT results for the new XLPE cable insulation are shown in table 1.

Table 1 isothermal OIT of new XLPE cable insulation:

temperature/. degree.C	180	190	200	210
					OIT/min	302	121	65	36

The relationship lnt-1/T was plotted from the Oxidation Induction Time (OIT) at four temperatures, and as shown in fig. 2, the activation energy E was found to be 127.6kJ/mol by linearly fitting the slope of the straight line, and the oxidation induction time at the operating temperature, i.e., the lifetime τ was extrapolated to 27.8 years. In this example, the working temperature was 70 ℃. Wherein t represents the oxidation induction time; t represents the absolute temperature, here the experimental temperature.

Wherein, the step S2 further includes: in thatThe DSC temperature rise curves for the new XLPE cable insulation (called new cable) and the field XLPE cable insulation (called field cable or field aged cable) were tested separately under nitrogen atmosphere, with the temperature rise range from room temperature to 140 ℃, and the results are shown in fig. 3. For crosslinked polyethylene insulation, the Kobayashi model parameter is a₁＝-0.056，a₂＝-0.022，b₁＝6.5，b₂＝-0.43，c₁＝1.1，c₂＝1.2，d₁＝-7.7，d₂-16. Equivalent thermal history parameters including thermal history temperature and corresponding thermal history time were calculated for new XLPE cable insulation and field XLPE cable insulation, respectively, using the Kobayashi model, and the results are shown in table 2.

TABLE 2 equivalent thermal history parameters for cable insulation

	Tp1(℃)	Tp2(℃)	Thermal history temperature T (. degree. C.)	Heat history time t (h)
					New cable	71.5	55.8	64.3	0.23
On-site cable	85.9	46.1	63.9	25809

The same mechanical relaxation phenomena of the high polymer can be observed at higher temperatures for shorter times (or higher frequency of action) and also at lower temperatures for longer times. Thus, increasing the temperature is equivalent to increasing the observation time for molecular motion and equivalent to viscoelastic behavior of the high polymer. This is the time-temperature equivalence principle.

Wherein, the step S3 further includes: according to the time-temperature equivalence principle, the thermal history time t of the field cable is compared₁Converted into a new cable thermal history temperature value T₀Time t of₁', and then subtracting the new cable thermal history time t₀The purpose is to eliminate the thermal history that occurs during cable manufacture. The conversion process is according to the following formula:

t″₁＝t′₁-t₀

in the formula T₁The in-situ cable thermal history temperature, E is the activation energy, and R is the gas constant. R is the gas constant of air and is equal to 8.314J/(mol.K).

Calculating to obtain corrected on-site aging cable thermal history time t₁″＝24451.5h。

Step S4 further includes:

s41: will t₁"further translated as the aging time at the operating temperature value, according to the following equation:

in the formula, t_sFor the operating temperature T of the cable_sAging time, E is activation energy, and R is gas constant; r is the gas constant of air and is equal to 8.314J/(mol.K).

The operating temperature of the known cable is T_sThe aging time t at the operating temperature is calculated at 70 ℃_s＝11479.3h。

S42: calculating the ratio of the aging time ts to the service life tau at the working temperature, and calculating the aging time t at the working temperature_sThe ratio of the aging state parameter to the service life tau is used as an aging state characterization parameter A (A is 0 to indicate non-aging, A is 1 to indicate end of service life) and is used for judging the aging degree of the on-site aging cable;

the aging state characterization parameters of field XLPE cable insulation are defined as follows:

A＝t_s/τ，A∈[0，1]

wherein tau is the service life of the new XLPE cable insulation at the working temperature.

If a is 0, the field cable is not aged, if a is 1, the field cable is terminated, and if 0 < a < 1, the field cable is in an aged state.

In this example, the calculated a-4.7% indicates that the field cable insulation is still in an early aged state. The cable sample is used for sampling the cable section in the station and is in low-load operation for a long time, and the judgment result objectively reflects the actual working condition of the cable, so that the method is proved to be effective.

The OIT test is carried out in the air atmosphere, so that the actual thermal-oxidative aging process of the cable insulation can be better simulated. In addition, the equivalent thermal history of the on-site aged cable is conjectured through DSC analysis, so that the defect that the aging age limit cannot reasonably reflect the aging state of the cable due to the difference of actual working conditions is overcome; moreover, the cable insulation field aging trend does not need to be obtained through long-term tracking detection, and a simple and rapid method for judging the insulation aging degree of the field aging XLPE cable is provided.